I think some (rich) people want the best IQ they can buy and they want all their lenses have the same"feel"to the picture (color rendering, micro contrast etc). So when Zeiss can give that, they will buy it.

And remember Zeiss is not after larfge numbers of sail, they are after high quality...(that is why they don't put OSS in their lenses as it may have bad influence on the IQ....